Saturn instrument unit

Saturn instrument unit

Components and layout
of a typical Saturn IB/V instrument unitThe instrument unit was designed by NASA's George C. Marshall
Space Flight Center in
Huntsville, Ala. IBM manufactured and assembled the instrument
unit at its Space Systems
Center in Huntsville.

Structure

Located between the Saturn S-IVB stage and the spacecraft.

Diameter:

21.7 feet

Height:

3 feet

Weight:

500 pounds; 4,400 pounds assembled

The structure consisted of three 120-degree sections of
thin-walled aluminum alloy face sheets bonded over sections
of aluminum honeycomb one inch thick.

Guidance and Flight
Control

Major Components: digital computer and data adapter; analog
flight control computer; inertial guidance platform;
emergency detection system control rate gyros and control
accelerometers. Together, they measured acceleration
and vehicle attitude, determined velocity and position,
and calculated and issued control commands to engine
actuators.

The inertial guidance platform sensed the vehicle's acceleration
and flight attitude. It sent measurements to the digital
computer through the data adapter. The digital computer
used the measurements to determine the vehicle's position
and velocity. Any corrections required to keep the vehicle
on course were calculated by the digital computer.

Correction signals, with outputs from control rate
gyros and control accelerometers, went to a switch selector.
The switch selector - one in the instrument unit and
one in each propulsion stage - decoded the correction
signals and passed them to the flight control computer.
The flight control computer issued commands to steer
the vehicle by gimbaling the engines.

Environmental Control

Heat from electronic components dissipated through 16 cold
plates lining the interior wall of the instrument unit.
An antifreeze-like coolant circulated from a reservoir
through the cold plates.

Instrumentation

Measurements: several hundred measurements were
made by sensors to monitor performance and environmental
data during flight. Results were telemetered to the
nearest ground station. Measurements were taken of:

Physical qualities, such as mechanical movements,
atmospheric pressures, sound levels, temperatures
and vibrations; all of which were transformed
into electrical signals.

Electrical values, such as voltage, current
and frequency, which determined stage separation,
engine cutoff and other flight control functions.

Telemetry: measurements were routed
to telemetry equipment for transmission to the ground.
Multiplexing - transmitting messages one after the other,
but so fast that they appear to be transmitted simultaneously
- was used on most channels. Multiplexing permitted
many transmissions on just a few channels.

Tracking: radar transponders onboard the launch vehicle
increased the range and accuracy of ground-based tracking
systems by replying to pulses of radar energy aimed at
the Saturn.

Radio Command: received and interpreted data transmitted
from the ground for the digital computer; followed a series
of error control steps to verify that the message received
agreed with the message transmitted.

Electrical System

Power Supplies:

Main supply: 20-30 seconds before liftoff, the launch
control center transferred power to four 28-volt alkaline
silver-zinc batteries.

Special supplies: five-volt master supply converted
the main supply to a reference voltage for the measuring
system's signal conditioning modules. The 56-volt power
supply provided the voltage for the guidance and control
system's inertial guidance platform.

Testing

Each major component was tested individually
and as part of its system (guidance and control, electrical,
etc.) before the instrument unit was shipped to the John
F. Kennedy Space Center.

After testing, the digital computer, data adapter, flight
control computer, inertial guidance platform and the
power supplies were removed and packaged separately for
shipment. The other components remained in the instrument
unit during shipment.